PURPOSE: Neonatal morbidity is strongly related to birth weight, and low birth weight (LBW) is known to be associated with childhood and even adult outcomes. This study aimed to investigate the associations of parental socio-economic status (SES) on the risks of low birth weight in the Republic of Korea. METHODS: Data on 450,574 singleton births were obtained from the National Birth Registration (NBR) database of 2008 and analyzed. Parental educational level and parental employment status were considered for the analysis. The place of birth, sex, marital status, parental age, and parity were included in the analysis of the unconditional multiple logistic regressions. LBW was defined as birth weight less than 2,500 g. RESULTS: 15,782 (3.5% of total subjects) of LBW infants were identified. The group with the lowest educational level (below high school) had the highest odds ratio of LBW, in both the father and mother, in the multivariable analysis [odds ratio (OR) 1.38 and 1.35, respectively]. Regarding paternal employment status, unemployed group had statistically significantly higher OR compared to the non-manual group (OR 1.14). Maternal employment status was analyzed not to have significant effect on the risk of LBW. CONCLUSION: The study confirmed that low parental SES in educational level or employment status increased the likelihood of LBW.